High-energy and inexpensive rechargeable battery systems based on earth-abundant materials are important for both mobile and stationary energy storage technologies. Rechargeable sodium-sulfur (Na—S) batteries able to operate stably at room temperature are among the most sought-after of these platforms because the cells take advantage of a two-electron-redox process to yield high storage capacity from inexpensive electrode materials. Realization of such batteries has been fraught with multiple stubborn problems ranging from unstable electrodeposition of sodium during battery recharge to rapid loss of the active cathode material by dissolution into the electrolyte.
The importance of rechargeable lithium batteries in portable electronics and their potential for electrifying transportation have been well described in several reviews. Various recent efforts have focused on the lithium-sulfur (Li—S) chemistry due to the high theoretical specific energy (2500 W h kg−1), high natural abundance and environmental benignity of the sulfur cathode, with great progress being achieved during the past decade. Although many technical challenges remain, the cost and feasibility of batteries that use metallic lithium as the anode and sulfur as the cathode appear good for applications in transportation, but less so for grid-related applications, where scale and cost are as important as gravimetric storage capacity. Sodium, the second lightest and smallest alkali metal is a low-cost alternative to lithium as anode and is available in regions all over world, it is unsurprising that interest in Na-based batteries predate those in Li-based ones.
High-temperature sodium-sulfur (Na—S) batteries operated at >300° C. with molten electrodes and a solid β-alumina electrolyte have been commercialized for stationary energy storage systems, confirming that this cell chemistry can meet the scale and cost requirements for feasibility in grid-scale applications. A stable room temperature (RT) analog of the rechargeable Na—S battery with a higher theoretical specific energy of 1274 W h kg−1 has to date proven elusive. The large difference in size between Na atom and Na+ ion defines one aspect of the challenge, as it is thought to make a sodium anode more prone than lithium to form unstable electrodeposits and dendrites. Sodium is also more reactive with aprotic liquid electrolyte solvents and forms a less stable protective solid electrolyte interphase (SEI) in aprotic liquids, which leads to lower electrochemical conversion efficiency and poor shelf life. Na+ ions are larger and less reducing than Li+ ions, which implies that transport and kinetics of electrochemical processes in the cathode are more sluggish. Finally, its reduction products with sulfur are more soluble than the analogous ones for lithium. Taken together, these traits mean that a successful Na—S cell must overcome multiple new challenges at both the anode and cathode, in addition to the already well-known ones facing Li—S batteries: the insulating nature of sulfur and its solid-state discharge product; the solubility of intermediate lithium polysulfides (LiPS) species and their associated shuttling between the electrodes, which lowers the Coulombic efficiency of the cell; and volume expansion of the cathode upon cell discharge. It is significant that some of these problems remain even when a solid-state electrolyte is employed in high-temperature Na—S cells in which the Na metal anode is a liquid.
Sulfur infused into microporous carbon materials with small pore sizes (dp<1.8 nm) and high surface areas (SA≥843 m2/g) have been reported previously. When employed as cathodes in Li—S cells, the materials have been reported to display only one of the two discharge plateaus observed in traditional Li—S batteries, which has been argued to lend support to the hypothesis that in microporous carbons sulfur undergoes a solid-state electrochemical reaction with Li+ directly forming solid sulfide product species in the cathode—i.e. without forming soluble LiPS. An alternative argument has been presented that supports formation of smaller sulfur (S2-4) species in microporous carbon substrates that upon reduction with Li+ cannot form soluble high-order LiPS. Although this argument is a reasonable interpretation of the electrochemistry data, support from thermodynamic analysis of the electrode has so far been lacking. Because the mechanism of cathode stabilization relies on changing the thermodynamics of reduced sulfur in carbon micropores to favor smaller sulfur species, this sets strict limits on the size of the carbon pores. On the anode side, strategies for preventing dendrite formation in lithium metal batteries are generally considered applicable for the sodium anode, but none have been demonstrated. Among the approaches that work for Li, efforts to reduce the magnitude of destabilizing electric fields near the anode by tethering anions to slow-moving or immobile supports, or those focused on introduction of LiF or LiF precursors (e.g. fluorinated polymer binders, salts or fluorinated electrolytes or electrolyte additives (e.g. ethylene carbonate carbonate) known to readily breakdown in the presence of Li to form LiF could potentially prevent dendrite proliferation in Na-battery systems.
Therefore, what is needed for rechargeable sodium-sulfur (Na—S) batteries able to operate stably at room temperature are robust strategies that protect both electrodes from degradation processes that are more severe than in the analogous Li—S batteries.